1. Field of the Invention
This invention provides for ferroelectric liquid-crystal displays having active matrix elements.
2. Description of the Related Arts
In addition to nematic and cholesteric liquid crystals, optically active tilted smectic (ferroelectric) liquid crystals have also been used recently in commercial display devices.
Clark and Lagerwall have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). On the basis of this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are fundamentally highly suitable for areas of application such as computer displays.
The use of FLCs in electro-optical or fully optical components requires either compounds which form tilted or orthogonal smectic phases and are themselves optically active, or the induction of ferroelectric smectic phases by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range.
The individual pixels of an LC display are usually arranged in an x,y matrix formed by the arrangement of a series of electrodes (conductor tracks) along the rows and a series of electrodes along the columns on the upper or lower side of the display. The points of intersection of the horizontal (row) electrodes and the vertical (column) electrodes form addressable pixels. The arrangement of the pixels is usually referred to as a passive matrix. For addressing, various multiplex schemes have been developed, as described, for example, in Displays 1993, Vol. 14, No. 2, pp. 86-93, and Kontakte 1993 (2), pp. 3-14. Passive matrix addressing has the advantage of simpler production and consequently lower production costs, but the disadvantage that passive addressing can only be carried out line by line, which results in the addressing time for the entire screen with N lines being N times the line addressing time. For usual line addressing times of about 50 microseconds, this means a screen addressing time of about 60 milliseconds in, for example, the HDTV (high definition TV, 1152 lines) standard, i.e. a maximum frame frequency of about 16 Hz, too slow for moving images. In addition, display of gray shades is difficult. At the FLC Conference in Brest, France (Jul. 20-24, 1997, see Abstract Book), a passive FLC display with digital gray shades was shown in which each of the RGB pixels (RGB=red, green, blue) was divided into sub-pixels, allowing the display of gray shades in digital form through partial switching. The disadvantage of this method is the considerable increase in the number of screen drivers necessary and thus in the costs (in the case of the above display, three times the number of drivers are necessary than in a standard FLC display without digital gray shades).
In so-called active matrix technology (AMLCDs), a nonstructured substrate is usually combined with an active-matrix substrate. An electrically non-linear element, for example a thin-film transistor, is integrated into each pixel of the active-matrix substrate. The nonlinear elements can also be diodes, metal-insulator-metal and similar elements, which are advantageously produced by thin-film processes and are described in the literature (see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein).
Active-matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-lane switching) mode. In each case, the active matrix generates an electric field of individual strength on each pixel, producing a change in alignment and thus a change in birefringence, which is in turn visible in polarized light. A severe disadvantage of this process is the poor video capability, i.e. excessively slow response times, of nematic liquid crystal.
For this reason amongst others, liquid-crystal displays based on a combination of ferroelectric liquid-crystal materials and active-matrix elements have been proposed, for example in WO 97/12355, Ferroelectrics 1996, 179, 141-152, or W. J. A. M. Hartrmann (dissertation, Eindhoven, 1992).
In the latter case, a combination of the so-called xe2x80x98quasi-bookshelf geometryxe2x80x99 (QBG) of an FLC and a TFT (thin-film transistor) active matrix is utilized and thus a high response speed, gray shades and high transmission are achieved simultaneously. However, the QBG is not stable over a broad temperature range, since the temperature dependence of the smectic layer thickness disrupts or rotates the field-induced layer structure.
Although the commercialized xe2x80x98chevronxe2x80x99 geometry (C1 or C2) used in commercial FLC displays has adequate temperature stability, it does not have sufficient brightness, since the effective tilt angle is well away from its optimum value.
The invention relates to an active-matrix liquid-crystal display (LCD) containing a ferroelectric liquid crystal which forms a xe2x80x98spontaneous bookshelfxe2x80x99 (SBG) geometry. This is not, as in the QBG, generated by application of electric fields and thus temperature-unstable, but, by contrast, is stable over a broad temperature range.
The SBG is formed according to the invention in the temperature region of the Sc* phase. For a definition of xe2x80x98bookshelfxe2x80x99 and xe2x80x98spontaneous bookshelfxe2x80x99 geometry, see, for example, J. W. Goodby et al, Ferroelectric Liquid Crystals, Gordon and Breach, Philadelphia, 1992, xe2x80x9cIntroduction to Ferroelectric Liquid Crystalsxe2x80x9d, and Mizutani et al in Conference Summaries, 6th Int. Conference on Ferroelectric Liquid Crystals, Brest, 1997, page 66.
The invention likewise relates to a ferroelectric active-matrix liquid-crystal display containing a ferroelectric liquid crystal, where the liquid crystal comprises one or more mesogenic compounds and where these mesogenic compounds contain a cyclic structure comprising two or more ring compounds and one or two side chains, wherein at least one mesogenic compound has at least one partially or perfluorinated side chain.
The ferroelectric active-matrix liquid-crystal display according to the invention is highly practicable, since it combines high transmission, a short response time, gray shades and a broad temperature range with one another.
Furthermore, the active-matrix FLC display according to the invention does not exhibit any xe2x80x98zig-zagxe2x80x99 deformations, or if it does, they are so slight that they are unimportant.